The present invention relates, in general, to the field of frequency shift keying (FSK) modulators and methods. More particularly, the present invention relates to a hybrid frequency shift keying modulator and method for generation of synthesized radio frequency (RF) binary FSK signals of the types commonly used, for example, in land-mobile digital radio communication systems.
FSK is a fundamental digital modulation mode of broad application throughout the communications industry, particularly in the field of land-mobile signalling as a means of selectively addressing radio receivers (binary-coded squelch), and for digital transmission of messages to radio paging receivers. A binary-code representing the receiver's address or the message to be sent is transmitted as a serial string of ones and zeros by shifting the transmitter carrier frequency to one of two discrete values within the channel. Among the popular digital paging formats are POCSAG, NEC/D3 and GSC. Motorola's "digital private line" or "DPL" system is an example of a binary-coded squelch format.
Signal generators used in testing these receivers have a unique set of conflicting requirements placed upon them. Data rates vary from essentially zero to several hundred bps, and future systems may operate in excess of 10 k bps. POCSAG systems typically dwell for long periods of time on a one or zero between messages, and DPL requires simultaneous voice band analog FM. In all systems, the original code waveform must be preserved to comply with system bandwidth limitations. Of course, no degradation of the generator's basic frequency stability or lock-up time is tolerable and no significant disturbances in the phase-locked loop (PLL) should occur during frequency transitions. Automated test equipment (ATE) systems often require that the signal generator automatically return to the channel center frequency after code transmission to allow testing of the analog sections of the receiver. Moreover, inclusion of FSK should require the fewest possible changes to the existing generator circuitry.
The modulated carrier must respond faithfully to the original modulating waveform over a wide range of data bandwidths extending to essentially DC without attendant synthesized carrier frequency drift or significant disturbances of phase-locked loops used in generation of the carrier. Existing signal generators offering FSK capability are unable to meet all of the above requirements.
Three methods of applying FSK to signal generators have traditionally been used, each having objectionable disadvantages. Perhaps the simplest scheme involves applying the FSK signal to the generator as direct FM. This method provides good reproduction of the high frequency components of the modulating waveform but cannot reproduce low frequency components which fall within the loop bandwidth of the system. Reducing the modulated phase-locked loop bandwidth to obtain an improvement in low frequency modulation response typically results in an objectionable increase in loop lock-up time.
Conventional two point modulation schemes help somewhat, but in any case, such systems can never dwell more than momentarily on a one or zero if phase lock is to be maintained. Low data rate restrictions of this approach can be overcome by unlocking one of the synthesizer loops and DC coupling the FSK signal to the free running VCO. This method, therefore, allows good reproduction of both high and low frequency waveform components. However, the carrier frequency will drift objectionably, requiring frequent retuning.
Another method alternates the synthesizer frequency control data between two values representing one and zero. This system allows arbitrarily low data rates, infinite dwell on one or zero, and maintains good carrier frequency stability. Unfortunately, the data rate is limited to a value within the loop bandwidth. When the loop breaks lock during a frequency shift, a lock-up transient is produced which cannot be eliminated, only shortened in duration by increasing the loop bandwidth. Increasing the loop bandwidth, however, would make analog frequency modulation more difficult. Furthermore, such a system is incapable of preserving the FSK signal wave shape.
Other methods include the use of a form of analog digital converter to modulate the programming of a frequency divider chain in the PLL to produce DC coupled FM. This method produces objectionable frequency drift however, and manufacturers of direct digital synthesizers often modulate the programming data of the device although these systems also do not preserve wave shape information.